… The wording “final play” makes it seem as if there’s more than one play needed….
A: If you have only one Play (or only one card to play, even if the rules allow more) then that one card would be both your first and your final play. So yes, there IS a final play, even if you’d only be playing one card.
So, in order to use Get On With It, you’d have to do it before your final play, i.e. before your ONLY play. You’d just not take your Play for that turn, and do Get On With It instead.